This invention relates broadly to mechanical fuel metering in fuel injected, internal combustion engines and, more particularly, pertains to an improved fuel metering pump which delivers a desired amount of fuel at a predetermined time and pressure.
Fuel metering pumps are mechanical supply devices used in conjunction with fuel injected internal combustion engines to increase fuel pressure for delivery to a direct fuel injector, and to meter an appropriate amount of fuel for each cycle of each cylinder in the engine. The fuel metering pump employs an internal piston which forces a smaller piston or plunger rod attached to it back and forth by using crankcase pulses existing in every two-stroke engine. The plunger rod expels fuel at a high-pressure which is achieved through the pressure area differential of the internal piston and the plunger rod. This fuel is delivered through a small diameter, high-pressure line to the direct fuel injector in the combustion chamber of the engine. Since the engine requires different fueling levels for different speed and load conditions, the stroke of the plunger rod must be adjustable. The correct quantity of fuel is determined by a small displacement of the plunger rod which results in injection at once per cycle. The amount of fuel injected at each cycle is controlled by varying the stroke of the plunger rod. This reciprocal motion is achieved through the engagement of a concentric button cam on the top of each plunger rod with a cam mounted for rotation on a camshaft which is connected to the external linkage of the throttle to receive driver demand. For each cylinder in the engine, there is a corresponding plunger rod and cam. The fuel metering pump utilizes a conventional stepper motor to act on the throttle linkage for start-up and idle control. A stepper motor is an electronically controlled, motive device that has its own plunger that can be moved in and out an incremental amount in response to the engine control module (ECM). The ECM receives signals from various engine sensors and changes fuel volume by sending a signal to the stepper motor so as to rotate the camshaft and its cam relative to the respective button cam on the top of each plunger rod. Rotating the cam against a strong throttle return spring limits the stroke that the plunger rod can move thereby limiting fuel quantity which is ultimately delivered at a high-pressure into an air space in the direct fuel injector. Here the fuel is mixed and starts to vaporize with air after which the fuel-air mixture is ignited in the combustion chamber.
While the fuel metering pump described above has been generally satisfactory at providing a stable idle that can maintain a set speed with a variable load, this design has been found to have several drawbacks. For example, it has been determined that the overall fuel requirements for an engine did not match the linear delivery characteristics of the metering pump. The engine required more fuel at acceleration and in mid-range speeds when a high load was placed on the engine than it required at wide open throttle. Also, stepper motor response and reliability were inadequate with the stepper motor mounted directly on the throttle linkage. Further, the stepper motor mounting used in the current fuel metering pumps subjects the stepper motor to dirt and corrosion which decreases the reliability and durability of the device.
Accordingly, it is desirable to provide a fuel metering pump which will deliver the proper quantity and pressure of fuel to the combustion chamber of a fuel injected, internal combustion engine at starting, idle and rapid acceleration or high load situations. It is also desirable to provide a fuel metering pump having a faster acting, more responsive stepper motor which allows for trimming the fuel level to the exact requirements for any throttle position. It is further desirable to provide a fuel metering pump having a cleaned sealed environment for the stepper motor to operate. It remains desirable to provide a fuel metering pump which permits simplification of the throttle linkage reducing cost, complexity and associated wear/service problems.